1. Field of the Invention
The present invention relates to a method and apparatus for roller charging, and more particularly to a method and apparatus for performing a charging process relative to an image carrying member in an electrophotographic image forming process.
2. Discussion of the Background
Conventionally, electrophotographic image forming apparatuses such as copying machines, printers, facsimile machines, and so on use a variety of ways for evenly applying a charge to a photoconductive member before generating an electrostatic latent image.
In one exemplary way, a corona discharge is used. A corona charge apparatus includes a charge wire made of tungsten or nickel and which is extended in a metal mesh casing. The charge wire is arranged at a position close to a photoconductive member, and a voltage of a direct current or a direct current overlaid with an alternating current is applied between the charge wire and the photoconductive member so as to produce a corona discharge therebetween. Thereby, the surface of the photoconductive member is charged.
The above corona charge apparatus, however, has a drawback in that various discharge products such as ozone, NOx, etc. are produced due to the relatively high voltage applied. This results in environmental pollution and also causes problems with the image forming process in that the discharge products often produce a coat of nitric acid or nitrate which adversely affects formation of the image.
Therefore, a contact type charge apparatus that produces less ozone and consumes less electricity has been used in place of the corona charge apparatus. Such contact type charge apparatus includes a charge member with a conductive material formed in a roller, a brush, or an elastic blade and which contacts a surface of an image carrying member such as a photoconductive member. The surface of the image carrying member is charged by an application of a voltage between the charge member and the image carrying member.
The roller charge member, for example, includes a metal core and an elastic layer (e.g., conductive rubber) covering the surface of the metal core. When such an elastic layer is left in contact under pressure with the surface of the image carrying member for a relatively long time period, an inclusion such as plastic included in the elastic layer may be extruded to the surface and will be deposited on the surface of the image carrying member. This results in a dirty mark on an image.
Further, in the contact type charge apparatus, the charge process is performed under the condition that the charge member contacts the surface of the image carrying member. Therefore, the charge member may receive the residual toner left on the surface of the image carrying member after an image transfer process. This causes a deterioration of charging performance.
As an attempt to solve the above problems, a roller of the charge roller included in the charge member is provided with spacers, tapes, or films on both ends thereof so that the both ends evenly have a slightly greater diameter. With this charge roller, the surface of the photoconductive member is held distant from the charge roller except for the ends thereof. Thereby, another non-contact type charge apparatus is made. Related techniques for this non-contact type charge apparatus are described in published Japanese unexamined patent application, Nos. 3-240076, No. 4-360167, No. 5-107871, for example.
Further, published Japanese unexamined patent application No. 7-121002, describes an image forming apparatus in which a photoconductive drum is wrapped at both ends with sheet members. A charged plate for applying a charge to the photoconductive drum contacts the sheet members fixed on the ends of the photoconductive drum, thereby charging the photoconductive drum. With such a configuration, an image forming surface of the photoconductive drum preserved between the two sheet members is held apart from the charged plate, while the surface is charged. This is referred to as a non-contact type charging.
With the above non-contact type charge apparatus, portions of the charge roller corresponding to an image forming region do not contact the surface of the photoconductive member. Therefore, it eliminates the drawbacks of the contact type charge apparatus such as the deposition of the material included in the elastic member on the photoconductive member and the transfer of the residual toner deposited on the photoconductive member to the charge member.
However, it is difficult to evenly increase diameters of both ends of the charge roller which is covered by the elastic member, by wrapping a film, for example, around the ends of the wrapping elastic member. This is because the film is needed to perfectly wrap each end surface of the elastic member without a slight gap, and therefore the variations of the film in length are needed to be avoided.
FIG. 31 shows the above-described charge roller in which each film 60 is short in a circumferential direction of the charge roller such that a space S is formed between edges of each film 60 facing each other after a turn. With the charge roller of FIG. 31, as illustrated in FIG. 32, a gap G held between the surfaces of the film 60 and a photoconductive member 4 loses a distance of δ in the above space S. That is, the gap G is not held constant during a turn of the charge roller.
In the above charge roller, the gap G is typically decreased around the center of the charge roller in its axial direction. This is caused due a deformation of the elastic member around the both ends thereof and so on. Therefore, when using a relatively thin film, the charge roller which is the non-contact type charge roller has a risk at the center portion thereof to contact the surface of the photoconductive member.
Accordingly, the film must be thick enough to avoid the above problem. However, an increase in film thickness will make the gap G greater, in particular, around the positions close to the films, resulting in abnormal discharge. This causes a dirty white mark on an image. In other words, operation of the charge roller is very sensitive to the thickness of the film.
Generally, in the non-contact type charging apparatus, somewhat constant charge voltage can be obtained under the condition that the gap is varied when the charge roller is applied with only the direct current voltage and when the gap between the surfaces of the charge roller and the photoconductive member (e.g., the image carrying member) is smaller than a predetermined value (i.e., 20 μm).
When the above gap is greater than the predetermined value, however, the charge voltage can no longer be constant and is decreased in accordance with the gap. To compensate such voltage reduction, a DC (direct current) voltage overlaid by an AC (alternating current) voltage is applied to the charge roller. Thereby, a constant charge voltage is produced.
In this case, abnormal discharge may occur when the voltage applied according to the gap is too large. The voltage to be applied is needed to be controlled at a level that does not cause an abnormal discharge. As a result, the gap needs to be smaller than a certain value at which abnormal discharge does not occur. In other words, the thickness of the film is also restricted from this aspect.
On the other hand, as illustrated in FIG. 33, the edges of the film 60 may be overlapped when the film has a length slightly longer than a length of a circumference of the elastic member 62. The overlapped portion has a thickness twice as great as the other portions of the film 60. Therefore, the gap between the surfaces of the charge roller and the photoconductive member is greater where the overlapped portions contact the photoconductive member in each turn of the charge roller. Thus, the above gap is changed in every turn of the charge roller either when the length of the film in the circumferential direction of the elastic member is made longer or shorter than the length of circumference of the elastic member.
Generally, the above-described photoconductive member (the image carrying member) includes a photoconductive drum having a drum shape. Accordingly, the film may be wrapped around both ends of the photoconductive drum in order to provide the gap between the surfaces of the charge roller and the photoconductive drum, as described above with reference to the published Japanese unexamined patent application No. 7-121002. In this case, the photoconductive drum is typically made of a hard material and therefore it will not be deformed when receiving pressure from the charge roller via the films. This in turn causes no deformation of the films.
However, in the case of the charge roller with wrapping films on its ends, the films are attached on the elastic member wrapped around the metal core. Accordingly, the elastic member is deformed by the pressure from the photoconductive member via the wrapped films and the films will accordingly be deformed. As a result, the films are easily peeled off. Therefore, in the charge roller with wrapping films, it is desirable to avoid an application of an intensive pressure to a specific portion of the elastic member so as not to cause a deformation and/or leaning toward that specific portion.
The above-described image forming apparatus described in the published Japanese unexamined patent application No. 7-121002, has a drawback. With this image forming apparatus, the film members are attached to the photoconductive drum side. In order to obtain a desired photocell charging performance, the distance of the image forming area between the film members needs to be longer. But, this makes it difficult to maintain the straightness of the relatively long surface of the photoconductive drum within a desired tolerance and therefore the manufacturing cost is increased. This drawback is further explained in detail in the following description.
In the above forming apparatus, a transfer process is achieved by contact between a transfer roller and the photoconductive drum. During this process, the transfer roller receives a higher pressure from the film members than other portions of the photoconductive drum. Thus, the transfer roller is prone to be worn at both ends and a leak of the charge at the ends thereof which are worn will occur. At the same time, the film members themselves will be worn and, as a result the gap for assuring the desirable charging performance cannot be formed.
Also, in the above forming apparatus, a cleaning process is achieved by contact between a cleaning member and the photoconductive drum. If the cleaning member has a length across both film members, the transfer roller receives a higher pressure from the film members than other portions of the photoconductive drum during the cleaning process. Therefore, the cleaning member is prone to wearing at both ends, causing leakage of the charge at the ends which are worn. At the same time, the film members themselves will also be worn and the charging process will also be degraded. That is, the cleaning member needs to have a length within a length between the both film members is arranged inside between the both film members.
The cleaning process is particularly needed in the non-contact type charging apparatus in which the charging member is arranged to face the photoconductive member with a relatively small gap because the residual toner can easily be transferred onto the charging member through this gap. If a cleaning member is not provided and the charging member is deposited with the residual toner, this causes a reduction of the charging performance and results in production of an abnormal image.
Therefore, to avoid the above problem, it is desirable to prevent the residual toner from flowing into the effective charging area by appropriately setting the effective cleaning width of the cleaning member. For this purpose, the effective cleaning width and the width of the effective charging area have the following relationship:                the effective cleaning width>the width of the effective charging area.        
The width of the effective charging area in the image forming apparatus is normally determined in the manner described below. First, the maximum size of a recording sheet acceptable by the image forming apparatus determines the width. When the size is A3, the length of its short side, 297 mm, is the width, and when the size is A4, the length of its short side, 210 mm, is the width. Second, based on the consideration of rolling of the recording sheet during the time of sheet transferring, an exposure width with a margin is determined. This exposure width may be varied based on the consideration of variations of sheet transferring quality between the machines and is normally a width of the short side length of the maximum sheet size plus a margin of 2 mm to 4 mm to both sides, resulting a width of 301 mm to 305 mm. As a feedback control, when a sensor pattern for measuring an image density, for example, is written in a side area outside the maximum sheet width, the writing width is accordingly increased.
Third, development width is wider than the exposure width so as to be able to develop images written inside the exposure width. The development width is, for example, 304 mm to 313 mm in machines capable of handling A3-sized recording sheets. Lastly, the effective charging width is determined. The effective charging width is wider than the development width because the voltage of the background in the development area is charged to a predetermined voltage. For example, the effective charging width is 305 mm to 322 mm in machines capable of using A3-sized recording sheets. Thus, the effective charging width and the associated values are determined according to accuracy of elements and assembling of each machine.
Blade, brush, and magnetic brush methods are widely known for cleaning the surface of the photoconductive member. In these methods, the cleaning member contacts the surface of the photoconductive member so as to mechanically scrape the toner, or the cleaning member is applied with a voltage to clean off the toner by an electrostatic force.
Accordingly, those types of the cleaning member which contact the surface of the photoconductive member are needed to be extended inside the both film members so as not to contact the film members under a consideration of the aforementioned problems. Therefore, as shown in FIG. 34, the effective cleaning width of the cleaning member satisfies Wa>Wb>Wc, wherein Wa represents an inside distance between the two film members, Wb represents the effective cleaning width, and Wc represents the effective charging width.
In the non-contact type charging apparatus in which the charge member and the photoconductive member are arranged close to each other with a small gap, straightness of the charge member and the photoconductive member is important. For example, when a charge roller is not very straight and is curved, for example, the charge roller will turn in an eccentric manner and the distance of the gap between the charge roller and the photoconductive member will vary. In some cases, a part of the charge roller will touch the photoconductive member during one turn of the charge roller. This is same to the straightness of the photoconductive drum.
Therefore, both the charge member and the photoconductive drum are required to be very straight, particularly between outside edges of the film members. Accordingly, if the length of such a charge roller or photoconductive drum is made shorter, this would increase yields of such components and thereby reduce the cost of manufacturing.
Based on the above, it would be preferable that the two film members are arranged with the shorter distance to each other and, in the non-contact charging apparatus, the inside distance between the film members is preferably equal to the effective charging width.
However, this will cause a problem in some cases. For example, FIG. 35 shows a case where two film members 218 are wrapped around both ends of a photoconductive drum 205. As shown in FIG. 35, if the film members 218 arranged at the positions drawn in virtual lines and the effective charging width Wc is between the inside edges of the two film members 218, the cleaning member contacts the film members 218 since the effective cleaning width Wb must be wider than the effective charging width Wc.
Therefore, in this case, the film members 218 are preferably arranged at the positions drawn in solid lines in FIG. 35. As a result, the distance between the two film members 218 is made longer. Accordingly, a relatively large area of the photoconductive drum 205 is required to be extremely straight.